A number of Mac owners are experiencing issues with iMessage and FaceTime logins after upgrading to the latest version of OS X 10.11.4 El Capitan released Monday, with evidence suggesting a bug in the apps' account activation system.

Mac owners have taken to Apple's Support Communities forum to report (1, 2, 3, 4) an apparent Messages and FaceTime authentication issue in OS X 10.11.4 El Capitan that prevents users from using either service. AppleInsider has also received multiple reports detailing identical problems. OS X 10.11.4 was released on Monday with various feature additions and improvements.

The issue presents itself after installing the latest version of OS X, with most users reporting a pop-up error dialogues reading, "Could not sign in to iMessage" and "An error occurred during authentication." In other cases, the iMessage setup window seems to send user ID information to Apple's servers, but hangs without timing out.

Since iCloud logins appear unaffected by the bug, the problem might trace back to Apple's iMessage and FaceTime control services. Reports of multiple errors within a single household or business deployment suggest a faulty node might be impacting a geographical subset of users. Circumstantial evidence supports the theory of a server-side issue, as at least one Mac owner saw login trouble persist after downgrading to OS X 10.10 Yosemite.

The percentage of Macs affected by the issue is unknown, though reports are coming in from both users of older model hardware and customers who just purchased their computer.

As for an effective workaround, users have attempted generating app-specific passwords, toggling two-factor authentication on and off, reinstalling the update and performing fresh installs, but a reliable alternative has yet to be discovered.

Some forum members who contacted Apple Support said representatives are aware of the problem and note Apple engineers are working on a fix. AppleInsider reached out for clarification on the matter and will update this article when an answer is received.

El Capitan's login issues come one day after iPad owners reported difficulty installing iOS 9.3 on older hardware. As with Messages and FaceTime on Mac, the issue on iPad appears during the account activation stage. However, the iPad bug prevents iOS from installing altogether, leaving some tablets unusable unless a system restore is performed through iTunes.